1. Field of the Invention
The present invention relates to a transformer with laminated sheet cores, and more particularly to a transformer for communication in which the sheet cores require a high .mu. value.
2. Description of the Related Art
In general, a transformer with laminated sheet cores consists of laminated cores, a coil bobbin and a coil whose whole portions except for terminal portions are connected together by means of varnish or the like. Several laminated core fixing structures other than such a resin fixing structure are disclosed in the following documents:
First, Japanese Laid-open Utility Model Application Publication No. 1-73909 discloses a fixing structure for securing laminated cores to a coil bobbin by tightly inserting, into the core hole of the coil bobbin, both the laminated cores and a holding plate having side walls for preventing the removal of the laminated cores.
Secondly, Japanese Laid-open Utility Model Application Publication No. 58-195405 discloses a structure in which a sheet core intentionally bent is disposed at the outermost end of the core hole of a bobbin. In this arrangement, the bent sheet core is resiliently pressed against the inner wall of the core hole of the bobbin. Thus, the cores are fixed to the coil bobbin.
Thirdly, Japanese Laid-open Patent Application Publication No. 60-133712 discloses a structure in which a semi-cured type resin sheet is wound around the leg portions of laminated sheet cores so as to follow their contour and a thermal shrinkage tape of fabric having a high strength is wound around the semi-cured type resin sheet so as to uniformly wrap the sheet, and then the thermal shrinkage tape is thermally cured. In this way, the cores are fixed to a coil bobbin.
Since the above mentioned prior art discloses the technique of fixing laminated cores to a core bobbin by pressing the laminated cores in the direction of their thickness, there may occur problems in that .mu. of Permalloy, in particular, is lowered and magnetic distortion increases because the seat cores are pressed. In order to compensate deterioration of characteristic, the number of laminated core sheets must be increased. This enhances the cost of material and the machining cost.